Semiconductor integrated circuit

ABSTRACT

A semiconductor integrated circuit includes a harmonic oscillator circuit, a first switch circuit configured to cause an oscillating state of the harmonic oscillator circuit to switch between an “on” state and an “off” state, a detector circuit configured to produce a voltage responsive to an amplitude of the oscillating output of the harmonic oscillator circuit, a decision circuit configured to detect whether the voltage produced by the detector circuit exceeds a threshold in synchronization with a clock signal, and a second switch circuit configured to control whether or not to apply noise from a noise source to the harmonic oscillator circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2015-046124 filed on Mar.9, 2015, with the Japanese Patent Office, the entire contents of whichare incorporated herein by reference.

FIELD

The disclosures herein relate to a semiconductor integrated circuit anda method of measuring noise.

BACKGROUND

One of the performance measures of an RF (radio frequency) circuit isnoise figure. Since noise figure relates to signal-receptionsensitivity, a product equipped with an RF circuit is subjected to noisefigure evaluation at the time of shipment in order to ensure that thenoise figure is within the specification range.

In the case of an automotive radar utilizing a 77-GHz band, for example,the noise of an RF circuit is measured in the high-frequency regionaround 77 GHz. Such measurement involves the use of an expensiveevaluation apparatus such as a spectrum analyzer for the purpose ofevaluating noise in high-frequency range, which leads to an increase inthe manufacturing cost. In order to reduce cost, it is desirable toprovide a mechanism for measuring noise levels inside an RF chip withoutresort to the use of an expensive evaluation apparatus.

Noise power levels are extremely low in the case of thermal noise atroom temperature. Measuring noise power thus involves the use of ahigh-gain and low-noise amplifier that can amplify minute noise signalsby a large factor. It is difficult, however, to provide such ahigh-quality amplifier embedded in a chip by using the inexpensiveSi-CMOS process. Conventionally, an amplifier module utilizing acompound semiconductor is provided as an embedded element in anevaluation device to perform noise evaluation.

-   [Patent Document 1] Japanese Laid-open Patent Publication No.    2004-318711-   [Patent Document 2] Japanese Laid-open Patent Publication No.    2001-154754-   [Patent Document 3] Japanese Laid-open Patent Publication No.    H10-240370

SUMMARY

According to an aspect of the embodiment, a semiconductor integratedcircuit includes a harmonic oscillator circuit, a first switch circuitconfigured to cause an oscillating state of the harmonic oscillatorcircuit to switch between an “on” state and an “off” state, a detectorcircuit configured to produce a voltage responsive to an amplitude ofthe oscillating output of the harmonic oscillator circuit, a decisioncircuit configured to detect whether the voltage produced by thedetector circuit exceeds a threshold in synchronization with a clocksignal, and a second switch circuit configured to control whether or notto apply noise from a noise source to the harmonic oscillator circuit.

According to an aspect of the embodiment, a semiconductor integratedcircuit includes a first noise detecting circuit and a second noisedetecting circuit, wherein each of the first noise detecting circuit andthe second noise detecting circuit includes a harmonic oscillatorcircuit, a first switch circuit configured to cause an oscillating stateof the harmonic oscillator circuit to switch between an “on” state andan “off” state, a detector circuit configured to produce a voltageresponsive to an amplitude of the oscillating output of the harmonicoscillator circuit, a decision circuit configured to detect whether thevoltage produced by the detector circuit exceeds a threshold insynchronization with a clock signal, and a second switch circuitconfigured to control whether or not to apply noise from a noise sourceto the harmonic oscillator circuit, wherein an oscillating frequency ofthe harmonic oscillator circuit of the first noise detecting circuit andan oscillating frequency of the harmonic oscillator circuit of thesecond noise detecting circuit are different from each other.

According to an aspect of the embodiment, a method for measuring noiseincludes initiating an oscillation of a harmonic oscillator circuit in afirst state in which noise from a noise source is not applied to theharmonic oscillator circuit, measuring, in the first state, a timelength from the initiation of the oscillation to a point in time atwhich an output of the harmonic oscillator circuit first exceeds apredetermined threshold, initiating an oscillation of the harmonicoscillator circuit in a second state in which noise from the noisesource is applied to the harmonic oscillator circuit, and measuring, inthe second state, a time length from the initiation of the oscillationto a point in time at which an output of the harmonic oscillator circuitfirst exceeds the predetermined threshold.

The object and advantages of the embodiment will be realized andattained by means of the elements and combinations particularly pointedout in the claims. It is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing illustrating an example of the configuration of aharmonic oscillator circuit;

FIG. 2 is a drawing illustrating the way the harmonic oscillator circuitoscillates in the case of no noise being applied;

FIG. 3 is a drawing illustrating the way the harmonic oscillator circuitoscillates in the case of noise being applied;

FIG. 4 is a drawing illustrating the way the time it takes for theharmonic oscillator circuit to reach a stationary state differsdepending on the noise level;

FIG. 5 is a drawing illustrating a method of measuring a noise level byuse of a harmonic oscillator circuit;

FIG. 6 is a drawing illustrating an example of the configuration of asemiconductor integrated circuit that measures a noise level by use of aharmonic oscillator circuit;

FIG. 7 is a drawing illustrating another example of the configuration ofa semiconductor integrated circuit that measures a noise level by use ofa harmonic oscillator circuit;

FIG. 8 is a drawing illustrating an example of the configuration of anLC oscillating circuit;

FIG. 9 is a drawing illustrating an example of the configuration of aring oscillator circuit; and

FIG. 10 is a drawing illustrating an example of the configuration of atransceiver inclusive of a noise detector.

DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the invention will be described withreference to the accompanying drawings. In these drawings, the same orcorresponding elements are referred to by the same or correspondingnumerals, and a description thereof will be omitted as appropriate.

FIG. 1 is a drawing illustrating an example of the configuration of aharmonic oscillator circuit. The harmonic oscillator circuit illustratedin FIG. 1 includes an amplifier circuit 10, a feedback circuit 11, anadder circuit 12, and a switch circuit 13. The switch circuit 13 causesthe oscillating state of the harmonic oscillator circuit to switchbetween the on state and the “off” state by causing the feedback pathforming a feedback loop to switch between the conductive state and thenonconductive state. The conductive state of the switch circuit 13serves to form the feedback loop, thereby placing the harmonicoscillator circuit in the oscillating state. The nonconductive state ofthe switch circuit 13 serves to break the feedback loop, thereby placingthe harmonic oscillator circuit in the non-oscillating state (inactivestate).

The adder circuit 12 adds together an input signal applied to an inputterminal IN and the output of the amplifier circuit 10 that is fed backthrough the switch circuit 13 and the feedback circuit 11, and appliesthe signal obtained by the addition to the input terminal of theamplifier circuit 10. The amplifier circuit 10 amplifies the inputsignal at the input terminal to output the signal obtained by theamplification at the output terminal. The gain of the amplifier circuit10 and the phase and gain of the feedback circuit 11 are set such as tocause the harmonic oscillator circuit to oscillate at desired frequency.It may be noted that the harmonic oscillator circuit oscillates ineither one of the two operating states, i.e., the operating state inwhich noise from a noise source subjected to evaluation is applied tothe input terminal IN and the operating state in which noise from thenoise source subjected to evaluation is not applied to the inputterminal IN.

FIG. 2 is a drawing illustrating the way the harmonic oscillator circuitoscillates in the case of no noise being applied. In the harmonicoscillator circuit illustrated in FIG. 1, only the natural thermal noiseis in existence when noise from the noise source subjected to evaluationis not applied to the input terminal IN. At a room temperature of 300 K,power P of the natural thermal noise is expressed in units of decibelsas follows:P=−174+10 log(Δf)  (1)Here, Δf is a bandwidth. The natural thermal noise, which exists all thetime even in the case of no noise from a noise source being applied, isamplified by the amplifier circuit 10 and fed back, resulting in beingamplified repeatedly in the feedback loop. Noise thus graduallyincreases as time passes, i.e., with the passage of each cycle in whicha signal propagates once around the feedback loop. As a result, theoutput voltage of the amplifier circuit 10 gradually increases.

In FIG. 2, a period Tt represents a transient period in which the outputvoltage of the amplifier circuit 10 continues to increase. A period Tsis a stationary period in which the output voltage of the amplifiercircuit 10 stays at a fixed level after having reached the saturationvoltage of the amplifier circuit 10.

FIG. 3 is a drawing illustrating the way the harmonic oscillator circuitoscillates in the case of noise being applied. In the harmonicoscillator circuit illustrated in FIG. 1, both the natural thermal noiseand noise from a noise source subjected to evaluation are in existencewhen the noise from the noise source is applied to the input terminalIN. These noises are amplified by the amplifier circuit 10 and fed back,resulting in being amplified repeatedly in the feedback loop. Thesenoises thus gradually increase as time passes, i.e., with the passage ofeach cycle in which a signal propagates once around the feedback loop.As a result, the output voltage of the amplifier circuit 10 graduallyincreases.

The transient period Tt in the case of noise being applied asillustrated in FIG. 3 is shorter than the transient period Tt in thecase of no noise being applied as illustrated in FIG. 3. This is becausethe input voltage level in the initial state as well as the inputvoltage level in each feedback cycle are higher in the case of noisebeing applied than in the case of no noise being applied. Namely, theamplifier circuit 10 at the time of a start is at a higher level in thecase of noise being applied than in the case of no noise being applied,and, also, a rate at which the voltage increases in each feedback cycleis also higher in the case of noise being applied than in the case of nonoise being applied.

FIG. 4 is a drawing illustrating the way the time it takes for theharmonic oscillator circuit to reach a stationary state differsdepending on the noise level. Waveforms 21 through illustrated in FIG. 4are the envelopes of oscillating signals of the harmonic oscillatorcircuit calculated by use of computer simulation. An LC oscillator isused as the harmonic oscillator circuit, with the oscillating frequencybeing 80 GHz and the loop gain being 0.2 dB. The waveform 21 is theenvelope of the oscillating voltage waveform in the case of theamplitude voltage of the noise input from the noise source being 500 μV.The waveform 22 is the envelope of the oscillating voltage waveform inthe case of the amplitude voltage of the noise input from the noisesource being 100 μV. The waveform 23 is the envelope of the oscillatingvoltage waveform in the case of the amplitude voltage of the noise inputfrom the noise source being 0 μV. To be more specific, the waveform 23is the envelope of the oscillating voltage waveform in the state inwhich only the natural thermal noise is present.

As can be seen from the simulation results illustrated in FIG. 4, ittakes approximately 34 nanoseconds for the amplitude of the oscillatingvoltage waveform to saturate and reach a stationary state in the case ofonly the natural thermal noise being present. In the case of theamplitude voltage of the noise input from the noise source being 100 μV,it takes approximately 18 nanoseconds for the amplitude of theoscillating voltage waveform to saturate and reach a stationary state.Further, in the case of the amplitude voltage of the noise input fromthe noise source being 500 μV, it only takes approximately 14nanoseconds for the amplitude of the oscillating voltage waveform tosaturate and reach a stationary state.

Accordingly, the measurement of a noise level is enabled by measuringthe time it takes for the amplitude of the oscillating voltage waveformto saturate and reach a stationary state from the time of initiation ofthe oscillating state of the harmonic oscillator circuit. Further, themeasurement of a noise level is also enabled by measuring the time ittakes for the amplitude of the oscillating voltage waveform to reach apredetermined voltage value from the time of initiation of theoscillating state of the harmonic oscillator circuit, without waitingfor the amplitude of the oscillating voltage waveform to saturate andreach a stationary state. For example, noise from a noise source thathas a known noise level may be input into the harmonic oscillatorcircuit, followed by measuring the time it takes for the amplitude ofthe oscillating voltage waveform to reach a predetermined voltage value.Such measurements may be taken with respect to a plurality of differentnoise levels. Subsequently, noise from a noise source that has anunknown noise level is applied to the harmonic oscillator circuit. Thenoise level can then be estimated by checking which one of thepreviously measured time lengths different from each other is theclosest to the time it takes for the amplitude of the oscillatingvoltage waveform to reach the predetermined voltage value.

Alternatively, the time it takes for the amplitude of the oscillatingvoltage waveform to reach the predetermined voltage value may bemeasured in advance in the presence of only the natural thermal noisewithout the inputting of noise from a noise source subjected tomeasurement. Thereafter, noise from a noise source (i.e., noise sourcesubjected to measurement) that has an unknown noise level may be appliedto the harmonic oscillator circuit. The time it takes for the amplitudeof the oscillating voltage waveform to reach the predetermined voltagevalue may then be measured. The noise level may be theoreticallyestimated based on this measured time length and the time lengthpreviously measured in the presence of only the natural thermal noise,as will be described later.

FIG. 5 is a drawing illustrating a method of measuring a noise level byuse of a harmonic oscillator circuit. In FIG. 5, the letter designation(a) denotes the output of the amplifier (i.e., the output of theamplifier circuit), and the letter designation (b) denotes the output ofa wave detector (i.e., the output of a wave detector circuit), with theletter designations (c) and (d) denoting a clock signal CLK and a datadetection result DATA, respectively, all of which are waveforms observedin the case of no noise being input from a noise source. Further, theletter designation (e) denotes the output of the amplifier (i.e., theoutput of the amplifier circuit), and the letter designation (f) denotesthe output of a wave detector (i.e., the output of a wave detectorcircuit), with the letter designations (g) and (h) denoting a clocksignal CLK and a data detection result DATA, respectively, all of whichare waveforms observed in the case of noise being input from a noisesource. Moreover, the letter designation (i) denotes a trigger signal TGthat causes the harmonic oscillator circuit to start oscillating. Themethod of measuring a noise level as illustrated in FIG. 5 will bedescribed after the configuration of a semiconductor integrated circuitfor measuring a noise level is described.

FIG. 6 is a drawing illustrating an example of the configuration of asemiconductor integrated circuit that measures a noise level by use of aharmonic oscillator circuit. The use of the semiconductor integratedcircuit illustrated in FIG. 6 enables the method of measuring a noiselevel illustrated in FIG. 5 to be performed to measure a noise level.

The semiconductor integrated circuit illustrated in FIG. 6 includes theamplifier circuit 10, the feedback circuit 11, the adder circuit 12, andthe switch circuit 13 as the harmonic oscillator circuit illustrated inFIG. 1. The semiconductor integrated circuit further includes a controlcircuit block 41, a clock generating circuit 42, a detector circuit 43,a decision circuit (DFF) 44, a capacitance element 45, a switch circuit46, and an unmeasured noise source 47. Boundaries between functional orcircuit blocks illustrated as boxes basically indicate functionalboundaries, and may not correspond to separation in terms of physicalpositions, separation in terms of electrical signals, separation interms of control logic, etc. Each functional or circuit block may be ahardware module that is physically separated from other blocks to someextent, or may indicate a function in a hardware module in which thisand other blocks are physically combined together.

Although the configuration of the semiconductor integrated circuitillustrated in FIG. 6 has the control circuit block 41 and the clockgenerating circuit 42 disposed inside the semiconductor integratedcircuit, this is only a non-limiting example. At least one of thecontrol circuit block 41 and the clock generating circuit 42 may bedisposed outside the semiconductor integrated circuit, thereby supplyinga clock signal to the semiconductor integrated circuit from an externalapparatus, or controlling the switch circuit 13 and the switch circuit46 from an external apparatus.

As in FIG. 1, the harmonic oscillator circuit includes the amplifiercircuit 10, the feedback circuit 11, and the adder circuit 12. Theswitch circuit 13 is a first switch circuit for causing the oscillatingstate of the harmonic oscillator circuit to switch between the “on”state and the “off” state. The switch circuit 13 may be regarded as partof the harmonic oscillator circuit, or may be regarded as a circuitelement external to the harmonic oscillator circuit. The control circuitblock 41 controls the switch circuit 13 as to the conductive state andnonconductive state thereof. As the control circuit block 41 places theswitch circuit 13 in the nonconductive state, the harmonic oscillatorcircuit stops oscillating. As the control circuit block 41 places theswitch circuit 13 in the conductive state, the harmonic oscillatorcircuit starts oscillating. A MOS transistor, for example, may be usedto implement the switch circuit 13.

The detector circuit 43 produces a voltage responsive to the amplitudeof the oscillating output of the harmonic oscillator circuit (i.e., theoutput of the amplifier circuit 10). Specifically, the detector circuit43 may be a wave detector circuit or rectifying circuit, and may performhalf-wave rectification or full-wave rectification by use of a diode orthe like, followed by smoothing the rectified waveform by use of asmoothing circuit or the like, thereby detecting the envelope of theoscillating output of the harmonic oscillator circuit.

The decision circuit 44 detects whether or not the output (i.e., outputvoltage) of the detector circuit 43 exceeds a threshold value insynchronization with the clock signal SLK. Specifically, the decisioncircuit 44 may be a flip-flop (D-flip-flop) that loads data insynchronization with the clock signal CLK. In the case of the flip-flopbeing used, the voltage level of the loaded data that exceeds apredetermined threshold level causes the loaded data to be “1”, and thevoltage level of the loaded data that falls below the predeterminedthreshold level causes the loaded data to be “0”. The rectifying circuitand the flip-flop are used to detect a point in time at which the loadeddata of the flip-flop becomes “1”, which enables the easy measurement ofa point in time at which the amplitude of the oscillating output voltageof the harmonic oscillator circuit reaches and exceeds the thresholdlevel.

The switch circuit 46 serves to control whether or not to apply thenoise from the unmeasured noise source 47 to the harmonic oscillatorcircuit. The control circuit block 41 controls the switch circuit 46 asto the conductive state and nonconductive state thereof. In the exampleillustrated in FIG. 6, the switch circuit 46 causes the path between theunmeasured noise source and a power supply circuit 48 to switch betweenthe conductive state and the nonconductive state. The nonconductivestate of the switch circuit 46 set by the control circuit block 41causes the unmeasured noise source 47 to be disconnected from the powersupply and to stop operating. As a result, noise from the unmeasurednoise source 47 is not applied to the harmonic oscillator circuit. Theconductive state of the switch circuit 46 set by the control circuitblock 41 causes the unmeasured noise source 47 to be connected to thepower supply to operate. As a result, noise from the unmeasured noisesource 47 is applied to the harmonic oscillator circuit. A MOStransistor, for example, may be used to implement the switch circuit 46.

The unmeasured noise source 47 may be coupled to the harmonic oscillatorcircuit through the capacitance element 45. The semiconductor integratedcircuit for measuring a noise level as illustrated in FIG. 6 is designedto be embedded in an RF chip or the like, and the unmeasured noisesource 47 may be an amplifier of the receiver circuit, for example. Ifthis amplifier is directly connected to the noise-level measuringcircuit illustrated in FIG. 6 all the time, such a connection may affectthe operation of the receiver circuit. In order to make the effect ofthe noise-level measuring circuit on the receiver circuit or the like assmall as possible, thus, it is preferable to electrically connect theunmeasured noise source 47 to the harmonic oscillator circuit throughcapacitive coupling by use of the capacitive element, rather thanproviding a direct wire connection.

In the following, a description will be given of the method of measuringa noise level by use of the noise-level measuring circuit illustrated inFIG. 6, with reference to FIG. 5 and FIG. 6.

The control circuit block 41 first makes the switch circuit 46nonconductive, thereby placing the unmeasured noise source 47 in a statein which no noise is generated. In this state, only the natural thermalnoise is applied to the harmonic oscillator circuit. In this state, thecontrol circuit block 41 asserts the trigger signal TG supplied to theswitch circuit 13, as illustrated in FIG. 5-(i), thereby placing theharmonic oscillator circuit in the oscillating state. Further, thecontrol circuit block 41 causes the clock generating circuit 42 togenerate the clock signal CLK in synchronization with the assertion ofthe trigger signal TG, as illustrated in FIG. 5-(c). With thisarrangement, the harmonic oscillator circuit amplifies the naturalthermal noise through the feedback loop, so that the amplitude of theoscillating output of the amplifier circuit 10 gradually increases asillustrated in FIG. 5-(a). The output of the detector circuit 43illustrated in FIG. 5-(b) is the voltage (e.g., the voltage of theenvelope) responsive to the amplitude of the oscillating output of theamplifier circuit 10, and gradually increases.

The decision circuit 44 detects the data indicated by the output voltageof the detector circuit 43 illustrated in FIG. 5-(b) at the risingedges, for example, of the clock signal CLK illustrated in FIG. 5-(c).Specifically, an output DATA of the decision circuit 44 illustrated inFIG. 5-(d) assumes “1” when the output of the detector circuit 43 at arising edge of the clock signal CLK is larger than a predeterminedthreshold TH, and assumes “0” when the output is smaller than thepredetermined threshold TH. In the example illustrated in FIG. 5, theoutput DATA of the decision circuit 44 illustrated in FIG. 5-(d) changesfrom “0” to “1” at a time t0.

The output DATA of the decision circuit 44 illustrated in FIG. 6 may beoutput to outside the semiconductor integrated circuit. The clock signal

CLK generated by the clock generating circuit 42 may also be output tooutside the semiconductor integrated circuit. An external apparatus maycount the number of pulses of the clock signal CLK from the first pulseto the later pulse at which the output DATA changes to “1”, therebydetecting the time it takes for the amplitude of the oscillating outputto reach the predetermined threshold TH from the initiation of theoscillating operation.

Alternatively, a counter may be provided inside the control circuitblock 41, and the clock signal CLK of the clock generating circuit 42may be supplied to the control circuit block 41, so that the countercounts the number of pulses. A signal indicative of the pulse count maythen be output to the outside. An external apparatus may identify thecount value observed at the time at which the output DATA changes to“1”, thereby detecting the time it takes for the amplitude of theoscillating output to reach the predetermined threshold TH from theinitiation of the oscillating operation. Alternatively, the output DATAof the decision circuit 44 may be supplied to the control circuit block41, which then identifies the count value observed at the time at whichthe output DATA changes to “1”. The control circuit block 41 may thenoutput the identified count value to the outside.

The generation of the clock signal CLK of the clock generating circuit42 does not have to start in synchronization with the assertion of thetrigger signal TG, and may start prior to the assertion of the triggersignal TG. In such a case, the time at which the oscillating operationof the harmonic oscillator circuit starts may be identified bymonitoring the trigger signal TG. When an external apparatus is used toidentify the time of initiation of the oscillating operation of theharmonic oscillator circuit, for example, provision may be made tooutput the trigger signal TG to outside the semiconductor integratedcircuit. The external apparatus may count the number of pulses of theclock signal CLK from the time of assertion of the trigger signal TG tothe time at which the output DATA of the decision circuit 44 changes to“1”.

After the time measurement as described above is performed in theabsence of noise from the unmeasured noise source 47, time measurementis further performed in the presence of noise from the unmeasured noisesource 47. The order of such time measurements may be reversed. Namely,time measurement in the presence of noise may be performed first,followed by performing time measurement in the absence of noise.

The control circuit block 41 makes the switch circuit 46 conductive,thereby placing the unmeasured noise source 47 in a state in which noiseis generated. In this state, the noise generated by the unmeasured noisesource 47 and the natural thermal noise are applied to the harmonicoscillator circuit. In this state, the control circuit block 41 assertsthe trigger signal TG supplied to the switch circuit 13, as illustratedin FIG. 5-(i), thereby placing the harmonic oscillator circuit in theoscillating state. Further, the control circuit block 41 causes theclock generating circuit 42 to generate the clock signal CLK insynchronization with the assertion of the trigger signal TG, asillustrated in FIG. 5-(g). With this arrangement, the harmonicoscillator circuit amplifies the noises through the feedback loop, sothat the amplitude of the oscillating output of the amplifier circuit 10gradually increases as illustrated in FIG. 5-(e). The output of thedetector circuit 43 illustrated in FIG. 5-(f) is the voltage (e.g., thevoltage of the envelope) responsive to the amplitude of the oscillatingoutput of the amplifier circuit 10, and gradually increases.

The decision circuit 44 detects the data indicated by the output voltageof the detector circuit 43 illustrated in FIG. 5-(f) at the risingedges, for example, of the clock signal CLK illustrated in FIG. 5-(g).Specifically, the output DATA of the decision circuit 44 illustrated inFIG. 5-(h) assumes “1” when the output of the detector circuit 43 at arising edge of the clock signal CLK is larger than a predeterminedthreshold TH, and assumes “0” when the output is smaller than thepredetermined threshold TH. In the example illustrated in FIG. 5, theoutput DATA of the decision circuit 44 illustrated in FIG. 5-(h) changesfrom “0” to “1” at a time td.

Noise is calculated as follows, for example, based on the time t0 andthe time td measured as described above. In the case of the roomtemperature being 300 K, the power of the natural thermal noise for abandwidth of 1 Hz is −174 dBm as previously described. With theimpedance of an output probing terminal 49 illustrated in FIG. 6designed to be 50Ω, a voltage Vini of the natural thermal noise isobtained as follows.Vini=(10^(−17.4)×50)^(0.5)=1.41×10⁻⁸  (2)In the presence of only the natural thermal noise as expressed by theabove formula, the number of generated pulses of the clock signal CLKmay be equal to N during the period from the initiation of theoscillation of the harmonic oscillator circuit to the time at which theoutput DATA of the decision circuit 44 changes to “1” upon the output ofthe detector circuit 43 exceeding the predetermined threshold TH. It mayfurther be assumed that the frequency of the clock signal CLK is equalto the oscillating frequency of the harmonic oscillator circuit, and avoltage value corresponding to the threshold TH is VTH, with the loopgain of the harmonic oscillator circuit being equal to G. In such acase, the following relationship is satisfied.VTH=(G ^(N) +G ^(N−1) +G ^(N−2) + . . . +G ² +G+1)Vini  (3)The reason why VTH is not equal to Vini multiplied by G^(N), but isexpressed as in the above formula is that the natural thermal noisehaving the voltage Vini is present all the time in addition to in theinitial state, and is added to the result of amplification obtained ateach cycle.

In the presence of the noise having a voltage Vn from the unmeasurednoise source 47, the number of generated pulses of the clock signal CLKmay be equal to M during the period from the initiation of theoscillation of the harmonic oscillator circuit to the time at which theoutput DATA of the decision circuit 44 changes to “1” upon the output ofthe detector circuit 43 exceeding the predetermined threshold TH. Thefrequency of the clock signal CLK may be equal to the oscillatingfrequency of the harmonic oscillator circuit, and a voltage valuecorresponding to the threshold TH may be VTH, with the loop gain of theharmonic oscillator circuit being equal to G. In such a case, thefollowing relationship is satisfied.VTH=(G ^(M) +G ^(M−1) +G ^(M−2) + . . . +G ² +G+1)Vn  (4)

It may be noted that Vn is sufficiently larger than Vini, so that theeffect of Vini is negligible.

Obtaining the loop gain G from the formula (3) allows the noise voltageVn of the unmeasured noise source 47 to be obtained from the formula (4)because VTH, G, and M are known. The noise voltage Vn obtained in thismanner is a voltage value corresponding to the power in a bandwidth of 1Hz.

It may be noted that the harmonic oscillator circuit amplifies frequencycomponents in the vicinity of the oscillating frequency while notamplifying other frequency components. As was previously described, theunmeasured noise source 47 subjected to noise level evaluation may be anamplifier of a receiver circuit, for example. The noise that may need tobe measured by use of the noise-level measuring circuit illustrated inFIG. 6 is a noise in the vicinity of the frequency at which theunmeasured noise source 47 operates during routine operations.Accordingly, the closed loop characteristics defined by the amplifiercircuit 10 and the feedback circuit 11 of the harmonic oscillatorcircuit may be set such that the harmonic oscillator circuit oscillatesat the frequency corresponding to the noise that may need to be measuredby use of the harmonic oscillator circuit.

FIG. 7 is a drawing illustrating another example of the configuration ofa semiconductor integrated circuit that measures a noise level by use ofa harmonic oscillator circuit. In the semiconductor integrated circuitillustrated in FIG. 7, a switch circuit 51 is provided in place of thecapacitance element 45 and the switch circuit 46 of the semiconductorintegrated circuit illustrated in FIG. 6.

The switch circuit 51 serves to control whether or not to apply thenoise from the unmeasured noise source 47 to the harmonic oscillatorcircuit. The control circuit block 41 controls the switch circuit 51 asto the conductive state and nonconductive state thereof. In the exampleillustrated in FIG. 7, the switch circuit 46 causes the path between theunmeasured noise source and the adder circuit 12 to switch between theconductive state and the nonconductive state. The nonconductive state ofthe switch circuit 51 set by the control circuit block 41 causes theharmonic oscillator circuit to be disconnected from the unmeasured noisesource 47, so that the noise generated by the unmeasured noise source 47is not applied to the harmonic oscillator circuit. The conductive stateof the switch circuit 51 set by the control circuit block 41 causes theharmonic oscillator circuit to be connected to the unmeasured noisesource 47, so that the noise generated by the unmeasured noise source 47is applied to the harmonic oscillator circuit. A MOS transistor, forexample, may be used to implement the switch circuit 51.

The control circuit block 41 causes the switch circuit 51 to switchbetween the conductive state and the nonconductive state in the mannerdescribed above so as to control whether or not to apply noise from theunmeasured noise source 47 to the harmonic oscillator circuit. With thisarrangement, the time it takes for the output voltage of the detectorcircuit 43 to exceed a predetermined threshold from the initiation ofoscillation is measured in both conditions, i.e., in the presence ofnoise from the noise source and in the absence of noise from the noisesource. The semiconductor integrated circuit illustrated in FIG. 7differs from the semiconductor integrated circuit illustrated in FIG. 6only in the circuit configuration for switching between the presence ofnoise input and the absence of noise input. Other configurations andoperations are the same or similar between these semiconductorintegrated circuits.

FIG. 8 is a drawing illustrating an example of the configuration of anLC oscillator circuit. The LC oscillator circuit illustrated in FIG. 8includes inductors 61 and 62, MOS transistors 63 through 65, andcapacitance elements 66 and 67.

The application of a HIGH signal to a control terminal 60 coupled to thegate of the MOS transistor 63 causes the MOS transistor 63 to becomeconductive, thereby causing the oscillating operation of the LCoscillator circuit to stop (i.e., to be set in the non-oscillatingstate). The application of a LOW signal to the control terminal causesthe MOS transistor 63 to become nonconductive, thereby causing theoscillating operation of the LC oscillator circuit to start (i.e., to beset in the oscillating state). One of the terminals 68 and 69 may beused as an oscillation output terminal, and the other one may be used asa probing terminal for the oscillation output. The LC oscillator circuitas illustrated in FIG. 8 may be used as a harmonic oscillator circuitillustrated in FIG. 6 or FIG. 7.

FIG. 9 is a drawing illustrating an example of the configuration of aring oscillator circuit. The ring oscillator circuit illustrated in FIG.9 includes n (n: odd integer) inverters 71-1 through 71-n and a MOStransistor 72.

The application of a LOW signal to a control terminal 73 coupled to thegate of the MOS transistor 72 causes the MOS transistor 72 to becomenonconductive, thereby causing the oscillating operation of the ringoscillator circuit to stop (i.e., to be set in the non-oscillatingstate). The application of a HIGH signal to the control terminal 73causes the MOS transistor 72 to become conductive, thereby causing theoscillating operation of the ring oscillator circuit to start (i.e., tobe set in the oscillating state). A terminal 74 may be used as anoscillation output terminal, and a terminal 75 may be used as a probingterminal. The ring oscillator circuit as illustrated in FIG. 9 may beused as a harmonic oscillator circuit illustrated in FIG. 6 or FIG. 7.

FIG. 10 is a drawing illustrating an example of the configuration of atransceiver inclusive of a noise detector. The transceiver illustratedin FIG. 10 includes antennas 81 and 82, an amplifier 83, a low-noiseamplifier 84, an intermediate frequency amplifier 85, a PLL(phase-locked loop) circuit 86, a mixer circuit 87, a noise detector 88,and a noise detector 89. This transceiver may be a radio-wavetransmitter and receiver circuit of an automotive radar, for example.

A transmission signal generated by the PLL circuit 86 is amplified bythe amplifier 83, and the amplified transmission signal is supplied tothe antenna 81, which transmits transmission waves. A wave signalreceived by the antenna 82 is amplified by the low-noise amplifier 84,and the mixer circuit 87 multiplies the amplified received signal by areference signal supplied from the PLL circuit 86. The signal obtainedby the multiplication is amplified by the intermediate frequencyamplifier 85, and the amplified signal is supplied to a signalprocessing circuit or the like disposed at the next stage.

The noise detector 88 coupled to the output of the low-noise amplifier84 may have the same or similar configuration as the noise-levelmeasuring circuit illustrated in FIG. 6 or FIG. 7. In this case, thelow-noise amplifier 84 serves as the unmeasured noise source 47illustrated in FIG. 6 or FIG. 7. The oscillating frequency of theharmonic oscillator circuit of the noise detector 88 may be designed tooverlap the frequency band of the output signal of the low-noiseamplifier 84, i.e., the frequency band of the RF (radio frequency)signals that are the received signals.

The noise detector 89 coupled to the output of the mixer circuit 87 mayhave the same or similar configuration as the noise-level measuringcircuit illustrated in FIG. 6 or FIG. 7. In this case, the mixer circuit87 serves as the unmeasured noise source 47 illustrated in FIG. 6 orFIG. 7. The oscillating frequency of the harmonic oscillator circuit ofthe noise detector 89 may be designed to overlap the frequency band ofthe output signal of the mixer circuit 87, i.e., the frequency band ofthe intermediate frequencies.

According to at least one embodiment, noise levels in a semiconductorintegrated circuit can be measured at low cost.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment(s) of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A semiconductor integrated circuit, comprising: aharmonic oscillator circuit; a first switch circuit configured to causean oscillating state of the harmonic oscillator circuit to switchbetween an “on” state and an “off” state; a detector circuit configuredto produce a voltage responsive to an amplitude of the oscillatingoutput of the harmonic oscillator circuit; a decision circuit configuredto detect whether the voltage produced by the detector circuit exceeds athreshold in synchronization with a clock signal; and a second switchcircuit configured to cause the harmonic oscillator circuit to oscillatein a first state in which noise from a noise source is not applied tothe harmonic oscillator circuit, and configured to cause the harmonicoscillator circuit to oscillate in a second state in which noise fromthe noise source is applied to the harmonic oscillator circuit.
 2. Thesemiconductor integrated circuit as claimed in claim 1, wherein thesecond switch circuit is configured to control whether to make or breaka connection between the noise source and the harmonic oscillatorcircuit.
 3. The semiconductor integrated circuit as claimed in claim 1,wherein the second switch circuit is configured to control whether ornot to apply power to the noise source.
 4. The semiconductor integratedcircuit as claimed in claim 1, wherein the detector circuit is arectifying circuit, and the decision circuit is a flip-flop.
 5. Asemiconductor integrated circuit, comprising: a first noise detectingcircuit; and a second noise detecting circuit, wherein each of the firstnoise detecting circuit and the second noise detecting circuit includes:a harmonic oscillator circuit; a first switch circuit configured tocause an oscillating state of the harmonic oscillator circuit to switchbetween an “on” state and an “off” state; a detector circuit configuredto produce a voltage responsive to an amplitude of the oscillatingoutput of the harmonic oscillator circuit; a decision circuit configuredto detect whether the voltage produced by the detector circuit exceeds athreshold in synchronization with a clock signal; and a second switchcircuit configured to cause the harmonic oscillator circuit to oscillatein a first state in which noise from a noise source is not applied tothe harmonic oscillator circuit, and configured to cause the harmonicoscillator circuit to oscillate in a second state in which noise fromthe noise source is applied to the harmonic oscillator circuit, whereinan oscillating frequency of the harmonic oscillator circuit of the firstnoise detecting circuit and an oscillating frequency of the harmonicoscillator circuit of the second noise detecting circuit are differentfrom each other.
 6. A method for measuring noise, comprising: initiatingan oscillation of a harmonic oscillator circuit in a first state inwhich noise from a noise source is not applied to the harmonicoscillator circuit; measuring, in the first state, a time length fromthe initiation of the oscillation to a point in time at which an outputof the harmonic oscillator circuit first exceeds a predeterminedthreshold; initiating an oscillation of the harmonic oscillator circuitin a second state in which noise from the noise source is applied to theharmonic oscillator circuit; and measuring, in the second state, a timelength from the initiation of the oscillation to a point in time atwhich an output of the harmonic oscillator circuit first exceeds thepredetermined threshold.